1. Field of the Invention
This invention relates to methods for the treatment and stabilization of municipal wastewater sludge, and, in particular, to methods for treating wastewater sludge cake with acid and calcium carbonate to produce a finished product, useful as a fertilizer, a soil conditioner or a synthetic soil component.
2. Description of the Background
Methods for the treatment of wastewater sludge to economically and safely produce a useful, soil-like end product, such as fertilizer, are needed in order to deal with the large amounts of wastewater sludge being produced by municipalities and industries. Alkaline stabilization and advanced alkaline stabilization are two conventional municipal wastewater sludge processes that have been used to produce an alkaline a soil-like end product used as a soil conditioner to control soil pH. Although the products of these processes may qualify as either Class A or Class B with regard to pathogen standards per 40 C.F.R. 503 Rule of the USEPA, the end products and processes have the negative characteristic of emitting significant amounts of ammonia and amines, such as trimethyl amine or diethyl amine both during processing as well as from the final product. As a result, the end products and processes for making them are clearly nuisances. For example, the laws of the State of New Jersey with regard to odor emissions make it very difficult to distribute such products for agricultural usage within the state.
The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides methods for treating wastewater sludge cake with acid and calcium carbonate or calcium carbonate-containing materials under acidic, low heat conditions to produce a granular or soil-like, finished product containing calcium carbonate.
The present invention modifies or improves conventional sludge stabilization processes in that the undesirable emission of ammonia and amines does not occur during processing or upon product completion or storage. The present invention also introduces a group of oxidant and non-oxidant odor control agents both prior to the addition of the acid and after the addition of the acid. These odor control agents include, for example, activated copper sulfate, ferrous sulfate, and other oxidants such as chlorine dioxide, stabilized liquid chlorine dioxide, hydrogen peroxide, sodium hypochlorite, ozone or combinations thereof. In addition, the invention produces a safe, odorless or nearly odorless product useful as a fertilizer, synthetic soil or soil conditioner for pH control as an ag-lime. This property is due at least in part to the calcium carbonate content of the final product. The invention also provides the advantage over traditional alkaline stabilization in that nitrogen contained within the ammonium and amines within the sludge is preserved in the product so that the nitrogen is available for agricultural use.
Accordingly, one embodiment of the invention is directed to a method for treating wastewater sludge to produce a stabilized product comprising the steps of mixing wastewater sludge comprising preferably between 10% and 40% solids with an acid to form an acid-sludge mixture, optionally adding an oxidant to assist with odor control of the mix, mixing the acid-sludge mixture with an amount of a material containing calcium carbonate, wherein the amount is such that the pH of the mixture does not exceed a safe handling pH of between 6 and 9, preferably no more than 8, and drying the mixture to produce the stabilized product. Generally, when the mixture is at least pH 7.0, the carbonate is in a carbonic acid form and will not control pH as equivalent to an agricultural limestone or ag-lime.
Another embodiment of the invention is directed to a method for treating wastewater sludge to produce a stabilized product comprising the steps of mixing wastewater sludge comprising preferably between 10% and 40% solids with an amount of a material containing calcium carbonate, wherein the amount is such that the pH of the mixture does not exceed about 7.0, mixing the calcium carbonate-containing sludge with an acid to form an acid-sludge mixture, and drying the mixture to produce the stabilized product.
Another embodiment of the invention is directed to a stabilized product made by the method of the invention.
Other embodiments and advantages of the invention are set forth in part in the description which follows, and in part, will be obvious from this description, or may be learned from the practice of the invention.